The present invention concerns a process for selective hydrogenation of diolefins concomitantly with isomerization of external olefins to internal olefins in liquid hydrocarbon cuts such as C.sub.4, C.sub.5, and C.sub.6 cuts from steam cracking or catalytic cracking which contain diolefins mixed with olefins. To make use of these olefins, they must be freed of diolefins by selective hydrogenation. In addition, it is occasionally advantageous to isomerize external olefins to internal olefins when they are of greater importance to downstream processes, for example alkylation or etherification units.
Such treatments (selective hydrogenation and isomerization) are carried out using catalysts which are based on at least one group VIII metal deposited on an amorphous or crystalline support. Palladium is currently used.
U.S. Pat. No. 4,724,274 describes a process for the preparation of 2-methyl-2-butene (internal olefin) from a C.sub.5 feedstock containing 2-methyl-1-butene (external olefin) and at least one n-pentene, using a catalyst which is based on a group VIII metal with a sulphur-containing compound present in the feedstock (H.sub.2 S, thiol, thiophene . . . ) at a concentration of 2 to 50 ppm. Linear olefins are selectively hydrogenated and 1-butene is isomerized. The process is carried out at a temperature of between 60.degree. C. and 120.degree. C. and a pressure of between 5 and 100 bars.
These palladium catalysts, however, do not isomerize external olefins sufficiently in the processes cited above. Even if the feedstock to be treated contains traces of sulphur, in order to increase the activity of the metal, a sulphur-containing compound must occasionally be introduced into the catalyst in the reactor (in situ) before bringing the catalyst into contact with the reactive feedstock.
This sulphuration (due to the presence of sulphur in the feedstock and/or in situ sulphuration of the catalyst) is, however, difficult to carry out since the sulphur-containing compound must be very well distributed over the whole of the catalytic bed in order to have a substantial effect on selectivity. In addition, this procedure can be long and costly; this leads to production losses or to hazardous operation depending on the sulphur-containing compounds employed.